The complex OsH(2)Cl(2)(P(i)Pr(3))(2) reacts with pinacolborane, Me(2)NH-BH(3), and (t)BuNH(2)-BH(3) to give the complexes OsH(2)Cl{eta(2)-HBOC(CH(3))(2)C(CH(3))(2)OBpin}(P(i)Pr(3))(2) and OsH(2)Cl(eta(2)-HBNR(1)R(2))(P(i)Pr(3))(2) (R(1) = R(2) = Me; R(1) = H, R(2) = (t)Bu) containing monosubstituted alkoxy- and amidoborinium cations coordinated as sigma-B-H ligands. The process is proposed to take place via the electrophilic 14-valence-electron fragment OsHCl(P(i)Pr(3))(2), which promotes hydride transfer from the corresponding borane to the osmium atom.